The study and synthesis of Group 4 transition metal complexes in Ziegler Natta catalysis

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THE STUDY AND SYNTHESIS OF GROUP 4 TRANSITION METAL

COMPLEXES IN ZIEGLER-NATTA

CATALYSIS

r

F--4

0

0

WARWICK

By

Jonathan P. Corden

A thesis submitted as part requirement for the degree of

Doctor of Philosophy

Department of Chemistry

University of Warwick

s/

T

f-

CONTENTS

Page

CHAPTER I Introduction 1

Group 4 transition metals 1

Titanium(IV) halides and their chemistry 3

Addition compounds of Titanium Tetrachloride 5

TiC14 Adducts with monodentate donor ligands 6

TiCl4 Adducts with bidentate donor ligands 9

Titanium(IV) Carboxylates and ß-Diketonates 12

Ziegler-Natta Catalysis 14

Stereoregulation in propene polymerisation catalysts 16

Supported ('Third Generation') catalysts 17

Group 4 Metallocene complexes and catalysis 18

MAO cocatalysts 21

Ansa-metallocenes 22

Alternative catalysts and ligand systems 26

Macrocyclic ligands 29

Schiff base ligands ₃₂

Ligand properties and conformational changes

34

Tetradentate Schiff base complexes of transition metals

35

Group 4 tetradentate Schiff base complexes

37

CHAPTER _{2 Tetradentate Schiff Base Ligands and their Group 4}

Metal Complexes.

₄₉

Introduction

₅₀

The stereochemistry of the complexes

51

Preparation of the SALEN-Type Schiff base ligands

and their complexes with Titanium(IV) and Zirconium(IV)

54

Page

Spectroscopic characterisation of the SALEN-Type Schiff base

ligands and their complexes with Titanium and Zirconium 58

Preparation of the SLPNDM Type Schiff base ligands and their

complexes with Titanium, Zirconium and Hafnium 71

Spectroscopic characterisation of the SLPNDM Type Schiff

base ligands and their complexes with Titanium, Zirconium and

Hafnium 74

Preparation of the CycH Type Schiff base ligands and their

complexes with Titanium and Zirconium 86

Spectroscopic characterisation of the CycH Type Schiff base

ligands and their complexes with Titanium and Zirconium 88

Modelling Studies 103

Experimental 109

CHAPTER 3 _{Reactions of Group 4 Tetradentate Schiff Base Complexes} 127

Introduction 128

Reactions of [M(L)C12] (M = Ti, Zr; L=

DMSALEN,

EtSALEN, PhSALEN) complexes with trimethyl aluminium 129

Spectroscopic properties of the products

133

Preparation of tetradentate Schiff base metal alkyl complexes of

A1(III) ₁₄₇

Spectroscopic properties of the products

148

Spectroscopic properties of the products from the reaction of

tetradentate Schiff base ligands and excess AlMe3

154

Attempted alkylation of [M(L)C12] by reaction with RMgX

157

Experimental ₁₅₈

CHAPTER _{4 Group 4 metal complexes of tetraazaannulene macrocyclic}

ligands

₁₆₂

Introduction ₁₆₃

Group 4 transition metal complexes 165

Preparation of the ligands and their Group 4 complexes

170

Spectroscopic properties of the products

174

Further reactions of [M(omtaa)C12] complexes

189

Experimental 190

CHAPTER _{5 Reactions of Pyridine-containing Teteraazamacrocycles and}

Group 4 Transition Metals 196

Introduction ₁₉₇

Preparation of the ligands Hpy and H2Mepy 198

Spectroscopic properties of the products 199

Reactions of H2Mepy with aluminium(III) alkyls 207

Spectroscopic properties of the products 208

Reactions of H2Mepy with Group 4 transition metals 212

Spectroscopic Properties of the products 213

Experimental ₂₁₅

CHAPTER _{6 Polymerisation Studies 219}

Introduction ₂₂₀

Ethylene polymerisation Studies 222

Styrene polymerisation Studies 228

REFERENCES ₂₂₉

APPENDIX _{X-ray Crystallographic Studies 241}

List of Figures, Tables, Schemes and Equations

Page

Chapter 1

Figu res

Figure 1.1 _{Cyclopentadienyl titanium dichloride [Cp2TiC12]} 4

Figure 1.2 A diagrammatic _{representation of the dimeric} 1: 1 adduct

[TiC14.

_THF]

6

Figure 1.3

A diagrammatic representation of the monomeric [TiC14.

NMe3]

adduct

7

Figure 1.4 A diagrammatic representation of cis-[TiC14.2POC13] 7

Figure 1.5 A diagrammatic representation of trans-[TiC14.2C5H5N] 8

Figure 1.6 A diagrammatic representation of [o-C6H4(CO2'Bu)2. TiC14] 9

Figure 1.7 _{The tetrameric [Ti4016] framework in [Ti(OR)4] compounds} 10

Figure 1.8 A diagrammatic representation of [TiC12(OPh)2] 11

Figure 1.9 _{The dimeric structure of [TiC14(acac)]} 13

Figure 1.10 _{Isomers of polypropylene} 14

Figure 1.11 _{The Cossee-Arlman mechanism} 15

Figure 1.12 _{The Chatt-Dewar-Duncanson picture of bonding in a metal-olefin}

complex (Arrows show the direction of electron flow) 16

Figure 1.13 _{The pathway of polymerisation suggested by Breslow and}

Newburg 19

Figure 1.14 _{The cationic titanium vinyl complex} 20

Figure 1.15 _{Showing the two low lying unoccupied orbitals dß and dir 20}

Figure 1.16 _{Showing the insertion step facilitated by an agostic interaction 21}

Figure 1.17 _{Showing rac- and meso- isomers of ansa-metallocenes 24}

Figure 1.18

_{A cyclopentad ienyl-amide ligand catalyst}

₂₆

Figure 1.19

_{A benzamidinato complex}

₂₇

Figure 1.20

_{Alternative ligand systems for a-olefin polymerisation}

₂₈

Figure 1.21

_{Example of a high dilution preparation}

₃₀

Figure 1.22

_{An example of the Richman-Atkins method}

₃₁

Figure 1.23

An example of a template synthesis

₃₁

Figure 1.24

The general method for Schiff base synthesis

₃₃

Figure 1.25

_{The enolimine tautomer (a), and the ketoamine tautomer (b) of}

H2SALEN 34

Figure 1.26

_{Possible arrangements of tetradentate Schiff bases in metal}

complexes

35

Figure 1.27

_{A representation of complexes formed with H2SALEN and}

H2SALOPHEN

36

Figure 1.28

_{Examples of unsymmetrical tetradentate Schiff base ligands}

36

Figure 1.29 _{Jacobsen's catalyst} 37

Figure 1.30 _{The molecular structure of [Ti(SALEN)C12]} 38

Figure 1.31 _{The molecular structure of [Ti(SALEN)C1(py)]} 38

Figure 1.32 _{The molecular structure of [Zr(SALOPHEN)2]} 39

Figure 1.33 _{The molecular structure of [Ti(ACEN)C12]} 40

Figure 1.34 _{The molecular structure of [Ti(ACEN)Cl(THF)]} 40

Figure 1.35 _{Possible reaction products from reactions with [Ti(SALEN)C12]} 42

Figure 1.36 _{The molecular structure of [Ti(SALEN)Me2]} 43

Figure 1.37 _{The molecular structure of [Hf(SALOPHEN)C12(THF)]} 44

Figure 1.38 _{The molecular structure of [Zr(ACEN)C12]} 45

Figure 1.39 _{The molecular structure of [Ti(MSAL)2C12]} 46

Tables

Table 1.1

_{Some properties of Group 4 metals}

2

Table 1.2

_{Oxidation states and stereochemistry of zirconium and hafnium}

₃

Table 1.3

_{Physical properties of titanium tetrahalides}

5

Schemes

Scheme 1.1 The _{enolisation and} ionisation _{of pentane-2,4-dione}

(acetylacetone) ₁₂

Scheme 1.2 _{The synthesis of cationic [(R6-ACEN)Zr(R')]+ complexes 48}

Equations

Equation 1.1 _{The preparation of anva-metallocenes 23}

Equation 1.2 _{Reaction of molecular oxygen and [Ti(ACEN)C1(THF)] 41}

Equation 1.3 _{Reactions of [Ti(SALEN)C12] (the arrows indicate the possible}

alkylation sites in the complex)

42

Equation 1.4

Showing the preparation of [M(L)C12(THF)] complexes

₄₄

Equation 1.6

_{The synthesis of [(R6-ACEN)Zr(R')2] complexes}

47

Chapter 2

Figures

Figure 2.1

_{A diagrammatic representation of (a) an ansa-metallocene, (b) a}

cis-MC12 Schiff base complex and (c) a trans-MC12 Schiff base

50

complex

Figure 2.2

_{A diagrammatic representation of a tetradentate Schiff base ligand}

showing the sites available for ligand modification (R, X, Y, Z)

51

Figure 2.3 _{The molecular structure of [Zr(ACEN)C12] 52}

Figure 2.4

_{Diagrammatic representations of the three types of tetradentate}

Schiff base ligand studied

53

Figure 2.5 _{The molecular structure of the free ligand H2SALEN 53}

Figure 2.6 _{A representation of the SALEN type Schiff base ligands 54}

Figure 2.7 _{The 'H N. M. R. spectrum of the free ligand H2EtSALEN 59}

Figure 2.8 Showing the symmetry in the SALEN ligands ₆₀

Figure 2.9 The 'H N. M. R. _{spectrum of the complex}

[Zr(DMSALEN)C12]. THF ₆₃

Figure 2.10 _{The fragmentation of SALEN type ligands 64}

Figure 2.11 _{The molecular structures of (a) H2DMSALEN, (b) H2EtSALEN}

and (c) H2PhSALEN

66

Figure 2.12 _{The molecular structure of [Ti(DMSALEN)C12] 68}

Figure 2.13 _{The molecular structure of [Ti(PhSALEN)C]2] 69}

Figure 2.14

_{Showing the coordination around the titanium atom in}

[Ti(PhSALEN)C12]

₇₀

Figure 2.15 _{A representation of the SLPNDM type Schiff base ligands 71}

Figure 2.16 _{The molecular structure of [VO(OMe)(SLPNDM)] showing its cis}

stereochemistry at the metal centre 72

Figure 2.17 _{The 'H N. M. R spectrum of H2SLPNDM 74}

Figure 2.18 _{The 'H N. M. R spectrum of the complex [Zr(PhSLPNDM)C12] 78}

Figure 2.19 _{The fragmentation of SLPNDM type ligands 79}

Figure 2.20 _{The molecular structure of the free ligand H2SLPNDM 81}

Figure 2.21 _{The molecular structure of [Ti(SLPNDM)C12] 82}

Figure 2.22 _{The molecular structure of [Zr(SLPNDM)C12(THF)]} 83

Figure 2.23 _{The molecular structure of the free ligands (a) H2EtSLPNDM and}

(b) H2PhSLPNDM 84

Figure 2.24 _{The superposition of the two molecules H2EtSLPNDM and}

H2PhSLPNDM 85

Figure 2.25 _{A representation of the CycH type Schiff base ligands} 86

Figure 2.26 _{The 'H N. M. R spectrum of the free ligand H2CycH} 88

Figure 2.27 _{Representation of the protons in CycH type ligands and their}

complexes 89

Figure 2.28 _{The 'H N. M. R spectrum of the complex [Ti(CycH)C121} 91

Figure 2.29 _{The fragmentation of CycH type ligands} 92

Figure 2.30 _{The molecular structure of the free ligand H2DMCycH} 95

Figure 2.31 _{The superposition of H2CycH and H2DMCycH with respect to the}

cyclohexane rings 96

Figure 2.32 _{The molecular structures of the free ligands (a)H2EtCycH and}

(b)H2PhCycH 97

Figure 2.33 _{The molecular structure of cis-[Ti(EtCycH)C12]} 99

Figure 2.34 _{Another view of the molecular structure of cis-[Ti(EtCycH)C]2]} 100

Figure 2.35 Showing _{the coordination around the titanium atom} in

[Ti(EtCycH)C]2] ₁₀₁

Figure 2.36 _{Diagram illustrating the structures of the ligands H2DMSALEN}

(left)and H2SLPNDM (right) 105

Figure 2.37

_{Overlay of the calculated minimum energy structure of}

trans-[Ti(DMSALEN)C12] with the structure determined by X-

Ray crystallography showing the atomic numbering system 105

Figure 2.38 Overlay _{of the calculated minimum energy structure of}

trans-[Ti(SLPNDM)C12J _{with the structure determined by X-Ray}

crystallography showing the atomic numbering system 107

Tables

Table 2.1

_{Summary of 'H N. M. R data (b / ppm) of SALEN type Schiff base}

ligands and their Group 4 complexes

61

Table 2.2

_{Summary of Proton decoupled 13C N. M. R data (b / ppm) of}

SALEN type Schiff base ligands and their Group 4 complexes

62

Table 2.3 _{Summary of the E. 1 and C. I spectra for SALEN type ligands and}

complexes

65

Table 2.4

_{Comparison of the mean values of the C-O, N-C, and C-C bond}

lengths (A) in free Schiff base ligands

67

Table 2.5

_{Selected bond angles (°) for [Ti(PhSALEN)C12]}

70

Table 2.6

Comparison of M-N, M-O and M-C1 bond lengths (A) in different

SALEN type complexes

71

Table 2.7 _{Summary of 'H N. M. R data (S / ppm) of SLPNDM type Schiff}

base ligands and their hafnium complexes 76

Table 2.8 _{Summary of proton decoupled 13C N. M. R data (S / ppm) of}

SLPNDM type Schiff base ligands and their hafnium complexes 77

Table 2.9 Summary of the E. 1 and C. I spectra for SLPNDM type free ligands

and their hafnium complexes 80

Table 2.10 _{Summary of the E. 1 spectra for CycH type free ligands 93}

Table 2.11 _{Summary of the E. 1 and C. I spectra for the titanium and zirconium}

complexes of the CycH type ligands

94

Table 2.12 _{Selected bond angles (°) for [Ti(EtCycH)C12] 101}

Table 2.13 _{Comparison of Ti-N, Ti-O and Ti-C1 bond lengths (A), and Cl-}

Ti-Cl bond angles (°) in different tetradentate Schiff base

complexes 102

Table 2.14

_{Comparison of the calculated minimum energies for the cis- and}

trans- isomers of titanium(IV) Schiff base complexes

104

Table 2.15

_{Comparison of selected bond lengths and angles determined by X-}

Ray crystallography for trans-[Ti(DMSALEN)Cl2]

_{with those}

predicted by molecular modelling (figures in parentheses) 106

Table 2.16

_{Comparison of selected bond lengths and angles determined by X-}

Ray crystallography for trans- _with those

predicted by molecular modelling (figures in parentheses)

108

Table 2.17

Summary of infra-red band frequencies of Group 4 tetradentate

Schiff base complexes

121

Table 2.18

_{List of the parameters used when customising the MM+ forcefield}

contained within HYPERCHEM

to incorporate the desired

transition metal ion-ligand interactions 124

Scheme

Scheme 2.1

_{Showing the synthetic pathways to [M(L)C12] complexes}

57

Chapter 3

Figures

Figure 3.1 Showing the possible reaction products from the reaction of

[M(L)C12] (M = Ti, Zr; L= DMSALEN, EtSALEN, PhSALEN)

with 2AlMe3 131

Figure 3.2

_{Showing the side reaction products obtained from toluene}

solutions

of

[Zr(DMSALEN)C12(A]Me3)2]

(a)

and

[Ti(EtSALEN)C12(AlMe3)2] (b) ₁₃₃

Figure 3.3

_{Showing the postulated structures of the reaction products}

₁₃₄

Figure 3.4

_{Possible structures for [Zr(EtSALEN)C12(AlMe3)2]}

135

Figure 3.5 _{The 'H N. M. R spectrum of [Zr(EtSALEN)C12(A1Me3)2] 138}

Figure 3.6 _{The '3C N. M. R spectrum of [Zr(EtSALEN)C12(A1Me3)2] 139}

Figure 3.7

Mass

_spectral

fragmentation

_pattern

_for

[Zr(DMSALEN)C12(AlMe3)2] ₁₄₁

Figure 3.8

_{The molecular structure of [A1Me(EtSALEN) A] (Me)2][A]MeCI3]}

₁₄₃

Figure 3.9 Showing _{the coordination around A12 (Al} in _cation) in

[A1Me(EtSALEN)A1(Me)2] [AIMeCl3]

₁₄₄

Figure 3.10 Showing _{the coordination around All (Al in cation) in}

[A]Me(EtSALEN)Al(Me)2][A]MeCl3]

₁₄₅

Figure 3.11 _{The molecular structure of [A]2(Me2)2(DMSALEN)A12(Me3)2] 146}

Figure 3.12 _{The molecular structure of [AIEt(SALEN)] 147}

Figure 3.13 _{Showing the splitting of the CH2-CH2 backbone resonance in}

Al(III) complexes of the substituted SALEN type Schiff bases

148

Figure 3.14

_{The 'H N. M. R spectrum of [AIMe(EtSALEN)]}

₁₅₀

Figure 3.15

_{The molecular structure of [(GaMe2)2(SALEN)]}

₁₅₂

Figure 3.16 _{The molecular structure of [A]2(Me2)2(DMSALEN)A12(Me3)2]} 154

Figure 3.17 _{Showing the two Al-Me resonances in the} 'H N. M. R spectrum of

the complex [(AlMe2)2(AIMe3)2(EtSALEN)] 155

Figure 3.18 _{The 'H N. M. R spectrum of [(AIMe2)2(AIMe3)2(EtSALEN)]} 156

Tables

Table 3.1 _{Summary of 'H N. M. R data (b / ppm) for [M(L)C12(A]Me3)2]}

complexes 136

Table 3.2

_{Summary of proton decoupled '3C N. M. R data (b / ppm) of}

[M(L)C12(A1Me3)2] complexes

137

Table 3.3

Selected

bond

_angles

_around

All

_and

A12

in

[A1Me(EtSALEN)Al(Me)2] [AIMeC13] 144

Table 3.4 Selected bond lengths _around All _and A12 in

[A1Me(EtSALEN)Al(Me)2] [AIMeCl3] 144

Table 3.5 1H N. M. R. of SALEN type Schiff base aluminium(III) alkyl

complexes in CDC13 149

Table 3.6 _{Summary of proton decoupled '3C N. M. R shifts (b/ppm. ) for}

SALEN type Schiff base aluminium(III) alkyl complexes 151

Schemes

Scheme 3.1

_{The postulated possible reaction scheme for the action of AIMe3}

on Schiff base complexes

129

Scheme 3.2

_{The reported products of the reaction of [Ti(SALEN)C12] with}

AIMe3

₁₃₂

Scheme 3.3

_{Showing the postulated reaction pathway for the formation of}

complexes, [(A1Me2)2(AIMe3)2(L)]

153

Chapter 4

Figures

Figure 4.1 Diagrammatic _{representation of the free ligands H2tmtaa and}

H2orntaa

Figure 4.2

_{The molecular structure of H2tmtaa showing its saddle-shape}

Figure 4.3

_{The molecular structure of [Ti(tmtaa)C12]}

163

165

167

Figure 4.4 _{The molecular structure of [Zr(tmtaa)(CH2Ph)2]} 168

Figure 4.5 _{The molecular structure of [Zr(tmtaa-Me)(Me)(THF)]} 169

Figure 4.6 _{A diagrammatic representation of the complexes involving the}

ZrX2 group and the ligands (a) Me4taen and (b) omtaa 170

Figure 4.7 _{The molecular structure of [(tmtaa)2Li4(DME)3]} 172

Figure 4.8 _{The 'H N. M. R. spectrum of [Zr(tmtaa)C12]. 2THF} 176

Figure 4.9 _{The 'H N. M. R. spectrum of [Zr(omtaa)C121} 177

Figure 4.10 _{The '3C N. M. R. spectrum of [Zr(omtaa)C12] in CDC13 solution} 178

Figure 4.11 _{The symmetry of the ligands H2tmtaa and H2omtaa confirmed by}

'H N. M. R spectroscopy 180

Figure 4.12 Showing the six '3C resonances expected for the ligand H2tmtaa 181

Figure 4.13

Showing

_the

_splitting

_of

_the

_aromatic

_{resonance in}

[Zr(tmtaa)C12].

_{2THF and [Zr(tmtaa)C12]}

₁₈₂

Figure 4.14 _{The molecular structure of [Zr(omtaa)C]2] showing its symmetry} 185

Figure 4.15 _{Another view of [Zr(omtaa)C12] showing its saddle-shape} 186

Figure 4.16 Showing _{the coordination geometry around zirconium} in

[Zr(omtaa)C121 ₁₈₇

Tables

Table 4.1

_{Summary of the 'H N. M. R. data (b/ppm. ) for the tmtaa and omtaa}

compounds 175

Table 4.2 _{Summary of proton decoupled '3C N. M. R shifts (b/ppm. ) for tmtaa}

and omtaa compounds

179

Table 4.3 _{Summary of the E. I. Mass Spectra of H2tmtaa, H2omtaa and their}

titanium(IV) and zirconium (IV) complexes 184

Table 4.4

_{Listing of the distance of M from the N4 plane (A) in [M(L)C12]}

complexes

187

Table 4.5

_{Selected bond angles (°) for [Zr(omtaa)C12]}

₁₈₈

Schemes

Scheme 4.1

_{Some of the reactions of [Ti(tmtaa)C12]}

Scheme 4.2

_{Showing the synthetic route for the preparation of H2tmtaa}

167

171

Chapter 5

Figures

Figure 5.1 _{The molecular structure of [(DMMepy)Ru(Cl)(CO)]} 197

Figure 5.2 _{Showing the pyridine based macrocycles (a) and (b)} 197

Figure 5.3 _{Showing the two ligands Hpy and Mepy} 198

Figure 5.4 _{The lH N. M. R. spectrum of the free ligand Mepy} 200

Figure 5.5 _{The '3C N. M. R. spectrum of the free ligand Mepy} 201

Figure 5.6 Showing the plane of symmetry in the ligands Hpy and Mepy 202

Figure 5.7 Showing the different protons in Hpy and Mepy 202

Figure 5.8 Showing the numbering scheme for the ligand carbon atoms 203

Figure 5.9 Showing the positions where bond breakage occurs in the ligands

Mepy and Hpy 204

Figure 5.10 _{The molecular structure of the free ligand H2Mepy} 204

Figure

_5.11 Another view of the molecular structure of H2Mepy 205

Figure 5.12 _{Another view of Mepy showing the direction of the N-H protons 205}

Figure 5.13 _{A representation of the two ligands H2Mepy and DMHpy. 206}

Figure 5.14 _{The 'H N. M. R. spectrum of the complex [(H2Mepy)A1Me3] 209}

Figure 5.15 _{The 'H N. M. R. spectrum of the complex [(HMepy)AIMe2] 209}

Figure 5.16 _{The 'H N. M. R. spectrum of the complex [(HMepy)A]Et2] 210}

Figure 5.17 _{The predicted structure of [(Mepy)ZrC12] 213}

Figure 5.18 _{The 'H N. M. R. spectrum of the complex [(Mepy)HfC12] 214}

Table

Table 5.1

_{Summary of proton decoupled 13C N. M. R shifts (b/ppm. ) for}

Mepy and its Al(III) complexes 211

Scheme

Scheme 5.1

_{Showing the route to the synthesis of the ligands Hpy and H2Mepy}

199

Chapter 6

Figures

Figure 6.1

_{Schematic representation of the high pressure polymerisation test}

rig

222

Figure 6.2

A diagrammatic representation of a tetradentate Schiff base ligand

showing the sites of ligand modification (R, X, ). 224

Figure 6.3 _{The temperature and pressure trace for [Ti(EtCycH)C12] during the}

polymerisation reaction 225

Figure 6.4 The GPC _trace for _{the polyethylene produced with}

[Ti(EtCycH)C12]. ₂₂₇

Tables

Table 6.1 _{Showing the results of ethylene polymerisation tests with Group 4}

complexes 223

Table 6.2 _{Results of the GPC analyses on the polyethylene products 226}

Table 6.3 Showing the results of styrene polymerisation tests with Group 4

complexes 228

ACKNOWLEDGEMENTS

The author would like to thank the following people for their help during the course of

this work.

Professor M. G. H. Wallbridge and Professor P. Moore, for their invaluable advice,

guidance and support throughout the past three years.

Dr I. R. Little, of BP Chemicals, for his assistance and help throughout this work.

Dr W. Errington, for his invaluable help and tuition in the art of X-ray crystallography.

Dr J. Hastings, for his assistance in obtaining some of the N. M. R. spectra reported in

this work.

Mr 1. K. Katyal, for his help by recording most of the mass spectra presented in this

thesis.

All the technicians in the Chemistry Department at the University of Warwick especially

John Haslop and Harry Wiles.

To the people on the third floor, especially Paul P, Damian and Jase from C304; Steve,

Sue and Satty from C303. Further thanks to Steve and Sue for their help with the

molecular modelling. To the 5th floor coffee people including Stevie D, Hutch, Rob and

Terry and finally to the chemistry football and cricket teams.

A very special thankyou to my wife Sally for all her support and encouragement as well

as a wonderful three years.

Finally BP Chemicals and the EPSRC for providing the funding for this work.

DECLARATION

All of the work described in this thesis is original and was, except where

otherwise indicated, carried out by the author.

Jonathan Paul Corden

August 1997

Some of the work described in Chapter 2 of this thesis has been published in the

following references:

2,2-Dimethyl-1,3-bis(N-salicylideneimine)propane

J. P. Corden, W. Errington, P. Moore and M. G. H. Wallbridge, Acta Crystallogr., Sect C, 1996,52,125.

2-[ 1-[(2-Amino-4,5-dimethylphenyl)imino]ethyl] _phenol

J. P. Corden, P. R. Bishop, W. Errington, P. Moore and M. G. H. Wallbridge, Acta C rystallogr., Sect C, 1996,52,2777.

Two Schiff Base Ligands Derived from 1,2-Diaminocyclohexane

J. C. Cannadine, J. P. Corden, W. Errington,

_{P. Moore and M. G. H. Wallbridge,}

Crystallogr., Sect C. _{1996,52,1014.}

Two Schiff Base Ligands Derived from 2,2-Dimethyl-1,3-propanediamine

J. P. Corden, W. Errington, P. Moore and M. G. H. Wallbridge, Acta Crystallogr., Sect Cl 1996,52,3199.

Two Schiff Base Ligands Derived from 1,2-Diaminoethane

J. P. Corden, W. Errington, P. Moore and M. G. H. Wallbridge, Acta Crystallogr., Sect C, 1997,53,486.

ABBREVIATIONS

Bu

_butyl

bipy _bipyridine

acac

_{pentane-2,4-dione}

Cp _{cyclopentadienyl}

THE _{tetrahydrofuran}

IR

Infra-red

MAO

_{methylaluminoxane}

N. M. R.

_{Nuclear Magnetic Resonance}

ppm parts per million

b

_{chemical shift in ppm}

13C

_{carbon 13 (b)}

1H

proton (b)

py

pyridine

Mes _{mesityl (2,4,6-CH3-C6H2)}

Me _methyl

Et

_ethyl

Ph

_phenyl

Bz benzyl

CO

_{carbon monoxide}

A _{Angstrom (1 A=1x 10-10 m)}

br

_broad

m _multiplet

d

_doublet

t _triplet

q

_quartet

qu quintet

M. W _{molecular mass}

EI Electron Impact Ionisation

CI Chemical Ionisation

FAB

_{Fast Atom Bombardment}

nm _{nanometres (1 nm =1x} 10-9 m)

x _{wavelength in nm}

M

_metal

TMA _{trimethyl aluminium}

R, R'

_{alkyl group}

L, L' ligand

X

halide

DMSO

dimethylsulphoxide

F. T. Fourier Transform

H2tmtaa _{5,7,12,14-tetramethyl dibenzo[b, i][ 1,4,8,11 ]-}

tetaazacyclotetradecine

H2omtaa 2,3,6,8,11,12,15,17-octamethyl-

5,14-dihydro-5,9,14,18-tetraazadibenzo-[a, h]-

tetaazacyclotetradecine

H2SALEN N, N'-Ethylenebis(salicylideneimine)

H2SLPNDM 2,2-Dimethyl-1,3-bis(N-salicylideneimine)propane

H2Hpy _{3,7,11,17-Tetraazabicyclo[ 1 1.3.1 ]heptadeca-1(17), 13,15-triene}

SUMMARY

In this thesis a study of Group 4 transition metal complexes and their possible use as

Ziegler-Natta catalysts for alkene polymerisation is described.

Several tetradentate Schiff base ligands have been synthesised and characterised,

including some previously unreported examples. The products obtained from the

reactions of Group 4 transition metal halides with the disodium salts of these ligands

have been isolated and identified. The stereochemistry of these complexes is important

for their use as Ziegler-Natta catalysts, and several complexes have been characterised by

X-ray crystallography. [Ti(DMSALEN)C12], [Ti(PhSALEN)C12], [Ti(SLPNDM)C12] and

[Zr(SLPNDM)C12(THF)] have a trans- geometry at the metal, and the complex

[Ti(EtCycH)C12] has a cis- geometry. The remaining complexes have been studied by

molecular modelling to establish a likely stereochemistry.

The reactions of these dichloro complexes with trimethyl aluminium have been studied to

gain insight into the chemistry involved in the polymerisation reactions. These reactions

proceed with retention of the two chlorine atoms yielding complexes of the type

[M(L)C12(A]Me3)2] (M = Ti, Zr; L= Schiff base). Side reactions also occur and the

by-products [A1Me(EtSALEN)Al(Me)2][A1MeC13] _{and [(A1Me2)2(AIMe3)2(EtSALEN)]}

have been isolated, and characterised by X-ray crystallography.

Work was also carried out on the synthesis of complexes of the type [MC12L] (M = Ti,

Zr; L= tetra-azamacrocycle) by the reactions of MC14 with the dilithium salt of the

ligand. These complexes have been fully characterised and the molecular structure of the

complex [Zr(omtaa)C12] determined by X-ray crystallography.

Finally all these complexes have been tested as Ziegler-Natta catalysts for ethene

polymerisation. A few selected complexes were also tested for their use in styrene

polymerisation and found to be inactive.

Ligands within this thesis

R _ý-ý RRR

-N N -N N-

OH HO OH HO

R= Me = H2DMSALEN R=H= H2SLPNDM

R= Et = H2EtSALEN R= Me = H2DMSLPMDM

R= Ph = H2PhSALEN R= Et = H2EtSLPNDM

R= Ph = H2PhSLPNDM

Ný NR

H

NNR

R=H= H2tmtaa

R= Me = H2omtaa

N

H -N N-H

iJ

H

Hpy

RQR

-N N

OH HO

R=H= H2CycH

R= Me = H2DMCycH R= Et = H2EtCycH

R= Ph = H2PhCycH

IN

H -N N-H

Me

H2Mepy

CHAPTER I

CHAPTER I

Introduction

The general objectives and aims of this research are to find compounds which

might act as homogeneous Ziegler-Natta catalysts, initially for the polymerisation of

ethene. These aims will be discussed in more detail later in the introduction (see

p. 28). Also discussed later are alternative compounds to the well-developed ansa-

metallocene homogeneous Ziegler-Natta catalysts (see p. 22). These ansa-

metallocene complexes are considered to be an `idealised' model for a catalyst with

regard to both steric and electronic effects at the metal centre. With this in mind,

ligand systems have been designed and synthesised so that upon complexation with a

metal the resulting stereochemistry at the metal centre is similar to that of the ansa-

metallocene complexes.

The compounds which have been synthesised and discussed in this thesis are

mainly Group 4 transition metal complexes, and usually halide-containing compounds.

What follows is a general introduction to Group 4 transition metal chemistry followed

by an introduction to Ziegler-Natta catalysis and the reasoning behind the work

described in this thesis.

Group 4 Transition Metals

The Group 4 metals, titanium, zirconium and hafnium

_{, are}

d-block elements,

each with four valence electrons. For example, titanium has the electronic structure

[Ar]3d24s2. The most stable and most common oxidation state of these elements, +4,

involves the loss of all these electrons. Titanium can also exist in a range of lower

oxidation states, most importantly Ti(III), (II), (0) and (-1). Zirconium and hafnium

show a similar range of oxidation states, but the tervalent states are much less stable

relative to the quadrivalent state compared with titanium. As a result, the

coordination chemistry of zirconium and hafnium is dominated by the oxidation state

IV, with only a small number of Zr(III) and Hf(III) complexes being known. Almost

are well characterised. One Zr(III) complex which has been characterised by X-ray

crystallography is the chlorine bridged dimer [{ZrCl3(PBu3)2}2]. '

Table 1.1

_{Some properties of Group 4 metals.}

Property Titanium Zirconium Hafnium

Atomic number 22 40 72

Atomic weight 47.88* 91.22 178.49*

Number of natural 5 5 6

isotopes

_{(48Ti; 73.9%)}

_{(90Zr, 51.5%)}

_{(180Hf; 35.2%)}

Electronic configuration [Ar]3d24s2 [Kr]4d25s2 [Xe]5d26s2

* Atomic weight reliable to ±3 in the last digit.

The most common coordination number of titanium is six (recognised for all

oxidation states of the metal), although compounds exist where the coordination

number is four, five, seven or eight. Titanium compounds in the III or lower

oxidation states are readily oxidised to the IV state, and titanium compounds can also

be hydrolysed to compounds containing Ti-O linkages.

Zirconium and hafnium prefer the higher coordination numbers, especially

eight. This preference for Zr(IV) and Hf(IV) follows from the relatively large size

and high charge of the +4 ions. These ions have the electronic configuration d° and

as a result there are no stereochemical preferences due to a partly filled d shell, which

results in a variety of coordination geometries.

The oxidation states and

stereochemistries associated with zirconium and hafnium are summarised in Table

1.2.

Table 1.2

_{Oxidation states and stereochemistry of zirconium and hafnium.}

Oxidation state

Geometry

_Example

M°

6

Octahedral

[Zr(bipy)3]

M' (d3)

_{Complex sheet and cluster structures}

Zr1(d2)

_{Complex sheet and cluster structures}

M11

8

[CpZrCI(dmpe)2]

Mm (d)

6

Octahedral

_{[Hf! 3]}

MN (d)

4

Tetrahedral

[Zr(CH2C6H5)a]

6

Octahedral

[Zr(acac)2C12]

7 Pentagonal Na3[MF7]

bipyramidal

7 _{Capped trigonal} (NH4)3 [ZrF7]

prism

8

Dodecahedral

[Zr(C2O4)4]¢

8

_{Square antiprism}

[Zr(acac)4]

The following section of this introductory chapter will aim to highlight some

of the chemistry and structural features of the Group 4 metals, especially titanium.

The topics discussed will, by necessity, be selective, and directed towards the

chemistry discussed in this thesis.

Titanium(IV) _{Halides and their chemistry}

The tetrahalides, especially the tetrachloride and tetrabromide, are all

powerful Lewis acids and form an extensive series of addition compounds with

2

neutral donors (Lewis bases). Most research on the tetrahalides has centred around

TiCl4, but it has been found that TiF4, TiBr4, and TiI4 form addition compounds that

are often isostructural to those of the tetrachloride. Titanium tetrachloride is a very

important starting material for much of the chemistry of titanium, especially in the

organometallic chemistry of the element. Titanium tetrachloride is, for example, used

for the synthesis of the organometallic compound bis-cyclopentadienyl titanium

dichloride [Cp2TiC12] by reaction with sodium cyclopentadienide.

Ti

cI

Figure 1.1 _{Bis-cyclopentadienyl titanium dichloride [Cp2TiCI2]}

[Cp2TiC]2] is a red, crystalline solid' and is the principal starting material for much of

the reported organometallic chemistry of titanium. The organometallic chemistry of

titanium has been covered in several detailed reviews.

4

Titanium tetrachloride is prepared in a number of ways, one way being the

treatment of titanium dioxide with chlorine gas in the presence of carbon. '

Ti02+2 C12+2 C

1000 OC

TiC14+2 CO

The remaining halides can be prepared from titanium tetrachloride and the

appropriate hydrogen halide. 2

11

TiC14 +4 HX TiX4 +4 HC1

(X=F, _{Br, I)}

All the tetrahalides are extremely moisture sensitive, with TiC14 fuming

copiously in air, and reacting vigorously with water to produce titanium dioxide.

TiC14+2 H2O

Ir" Ti02+4HCl

Due to this hydrolysis, reactions carried out using titanium tetrahalides must

be performed under a dry atmosphere. Some properties of the titanium tetrahalides

are shown in Table 1.3.

Table 1.3 _{Physical properties of titanium tetrahalides}

Compound _{Colour and}

physical state

m. p(°C) b. p(°C) Structure*

TiF4 _{White crystalline}

- 284(subl) Fluorine bridged

solid

polymer

TiCl4 Colourless liquid

-24.1 136.45 Tetrahedral

monomer

TiBr4 _{Orange crystalline} 38.25 233.45 Tetrahedral

solid

monomer

TiI4 _{Dark brown solid} 155 377 Tetrahedral

monomer

* Data from ref 6

Addition Compounds of Titanium Tetrachloride.

When discussing the addition compounds of the Group 4 metals thought

should be addressed to the ligand donor atom and the metal centre. In oxidation state

0; Ti°, Zr° and Hf' are soft acids and as such would be expected to form adducts with

soft bases such as CO, PR3, R3As, R2S, etc. An example of one such complex is

[Ti(CO)2(PF, )(dmpe)2]. When in oxidation state +4, these metal centres are hard

acids and therefore would be expected to form addition compounds with hard bases

e. g. OH-, RO-, NH3, RNH2, F, Cl-, etc. Many examples of such compounds are

known e. g. [TiCI6]2" and [Ti(acac)2C12]. However, even these hard M4+ centres can

also form adducts with soft bases e. g. [TiC14(diars)2] and [TiC14.2PMe3].

Thus TiCl4 forms adducts with oxygen, nitrogen, sulphur, phosphorus and

arsenic donor ligands. In most of these addition compounds the titanium is in an

octahedral environment, although coordination numbers of 5,7 and 8 have also been

cited.

2

TiC14 Adducts with Monodentate Donor Ligands.

1: 1 Adducts Giving [TiC14. LJ (L = monodentale ligand)

These compounds are well documented and a large number of these have been

fully characterised by X-ray diffraction. In these compounds the titanium is often in

an octahedral environment by dimerisation through halogen bridges. This can be seen

in the THE adduct.

CI

cl

fI

cl

fI

cl

cl

_C,

Öc'

Figure 1.2

_{A diagrammatic representation of the dimeric 1: 1 adduct [TiC14. THF]}

Two types of metal-chlorine stretching vibrations are evident in the IR spectra

of these compounds. These are Ti-Cl terminal stretches (in the region 450-350cm ')

and Ti-Cl-Ti bridging vibrations (in the region 300-200cm ').

One notable, and interesting, exception to the dimeric formulation is the

trimethylamine adduct [TiC14.

NMe3]. This is monomeric, containing five-coordinate

trigonal bipyramidal titanium. '

CI

CI

Ti -Cl

CIý

NMe3

Figure 1.3 A diagrammatic representation of the monomeric [TiC14. NMe3] adduct

Since this work Everhart and Ault' have studied the reactions of TiCl4 with NH3 and

(CH3)3N. Cryogenic thin film experiments with these reaction products followed by

subsequent warming have led to the formation of interesting amido and imido

complexes.

1: 2 Adducts Giving [TiCl4.2L] (L = monodentale ligand).

Several of these adducts have been characterised by X-ray diffraction to

reveal monomeric, hexa-coordinate titanium species with the donor ligands in a

cis-configuration, for example where L= POC13,

MeCN, and Et20.9'1°'11

cl

cif

cl

/-P

CI

CI

P

CI

cl

Figure 1.4 A diagrammatic representation of cis-[TiC14.2POC13]

The bis-THF adduct [TiC14.2THF] is a yellow crystalline solid which is relatively easy

to prepare. 12 [TiC14.2THF] is a convenient alternative to TiC14 as a starting material

in the synthesis of many titanium complexes.

Although _{the cis-configuration} is seen in the majority of cases, the

trans-structure is also possible. Research with more sterically hindered ligands has

shown that the possibility of forming the trans- form tends to increase as the steric

bulk of the ligand increases.

Two complexes which have been isolated and characterised as having the

octahedral geometry with trans-donor ligands are [TiC14.2PhCO2Et]

13 and

[TiC14.2C5H5N].

'4

CI\ CI

CI

I

CI

Ü

Figure 1.5 A diagrammatic representation of trans-[TiC14.2C5H5N]

Certain ligands, such as POC1315'9

and EtOAc,

16

may produce 1: 1 and 1: 2

adducts depending on the reaction conditions. The 1: 2 adducts are prepared by

distilling TiC14 into a flask in which excess ligand has been introduced, and any

unreacted ligand is then distilled off after the reaction has been stirred for one hour.

The 1: 1 adducts are prepared by mixing (e. g. EtOAc and TiC14) in a 1: 1 molar ratio at

liquid nitrogen temperatures, on warming crystals of the 1: 1 adduct are formed.

Other ligands such as ketones and acid halides appear to form 1: 1 adducts

exclusively. It is clear that there is a fine balance between both steric and electronic

effects which influence the stoichiometry, and also the structure, in the formation of

1: 1 and 1: 2 adducts. " It is relevant to note here that the possibility of the existence

of cationic titanium species in some reactions cannot be overlooked. Indeed there are

many well-characterised cationic species, such as [CpTi(MeCN)5][SbCI6]3.2MeCN,

18

and although they will not be discussed in detail here, reference to them will be made

as appropriate in the various parts of the thesis.

TiC14 Adducts with Bidentate Donor Ligands.

1: 1 Adducts giving [TiCl4. B] (B = Bidentate Liganci')

The resulting adduct is generally monomeric with the titanium in an octahedral

environment. This has been shown using X-ray crystallography, with ligands such as

B= Me2C(COMe)219 and o-C6H4(CO2'Bu)2.2°

cl

i ýci

Ti

O

I\O

CI

/,

Figure 1.6

_{A diagrammatic representation of [o-C6H4(CO2'Bu)2. TiCl4]}

Titanium(IV) Compounds from TiCI4

Titanium(IV) Alkoxides.

As well as forming neutral adducts, TiCl4 reacts with a variety of compounds

with the replacement of one or more chlorine atoms. The best studied of these

compounds are the titanium(IV) alkoxides. A detailed review of this chemistry has

been reported by Bradley. 21 There are two general preparative routes for

titanium(IV) alkoxides available:

(a)TiC14 +4 NaOR alcohol _{, [Ti(OR)4] +4 NaCl}

(b)TiC14 +4 ROH +4 NH-3(anhydrous)

[Ti(OR)4] +4 NH4C1

(R = alkyl, aryl)

Method (b) is normally employed because in the absence of a reagent that will

remove the HCI, the reaction will only proceed as far as the [TiC12(OR)2] derivative.

Several alkoxy systems have been isolated and structurally characterised, for

example [Ti(OMe)4], 22 [Ti(OEt)4]23 and [Ti(OMe)(OEt)3].

24

They exist as tetramers

in the solid state and have a [Ti4016] framework which demonstrates the preference of

titanium for six fold coordination.

0

₀

O/

I I\O/

Ti

I \O

o\Ti/O\Ti

O/

I \O/ I \O

00

O=OR

Figure 1.7 _{The tetrameric [Ti4016] framework in [Ti(OR)4] compounds}

In solution, the lower alkoxides have been found to be trimeric. However

they are proposed to be monomeric if sterically hindered by a bulky alkyl group. Due

to the steric bulk of the phenoxides, [Ti(OPh)4], such species readily form 1: 1 adducts

whereas the sterically unhindered methyl and ethyl alkoxides [Ti(OR)4] (R = Me, Et)

do not form 1: 1 adducts. 25 This difference in behaviour towards Lewis bases is

related to the monomeric, and therefore coordinatively unsaturated, nature of the

phenoxides in solution, in contrast to that of the alkoxides.

The lower chain alkoxides are readily hydrolysed by moist air. However, the

higher homologues, and the phenoxides are much less susceptible to hydrolysis. With

carefully controlled conditions it is possible to isolate polymerisation intermediates.

Klemperer and co-workers have carried out controlled hydrolysis reactions to give

the polyalkoxides [Ti704(OEt)20], [Ti806(OPh)20] and[Ti, 008(OEt)24] which have

been characterised by X-ray diffraction. 26

A variety of [TiX4_,,

(OR)] (R = alkyl, aryl, alkenyl; X= halogen; n=1,2,3)

species are known. All of these compounds are hygroscopic. The compounds

[TiC12(OPh)2]27 and [TiC]2(OEt)2]28 have been structurally characterised by X-ray

crystallography. This reveals that both compounds are dimeric containing penta-

coordinate titanium in a trigonal bipyramidal environment. These compounds are

generally prepared by direct reaction between the parent tetra-alkoxide and the

appropriate molar proportion of the tetrahalide.

CI

0 _/O

CI Ti Ti CI

/ --ý

O/ 0

CI

Figure 1.8

_{A diagrammatic representation of [TiC12(OPh)2)}

Titanium(IV) _Carboxylates

TiC14 reacts with both aryl and alkyl monocarboxylic acids to produce

substituted species with the elimination of hydrogen chloride gas. Ideally the addition

of stoichiometric amounts of acid to the reaction system should allow all four

substituted titanium compounds to be prepared by the successive replacement of

chloride ions by carboxylate ligands. This is shown in the equation below

TiC14 +x RCO2H

[TiC14_x(O2CR).,

] +x HCl

(R = alkyl, aryl; x=1,2,3,4)

From the equation we see that the first product arising from the elimination of

one mole of hydrogen chloride is [TiC13(O2CR)]. Further work is required to

establish whether the further substituted products can be obtained, since competing

oxygen abstraction reactions can also occur.

Titanium(IV) ß-Diketonates

Another type of ligand which contains an OH group are the ß-diketonates.

Of this class of bidentate chelate ligands the most commonly used is pentane-2,4-

dione (acetylacetone, acac). This forms an anion as a result of enolisation and

ionisation, as shown in the scheme below, and it is the enolate form which forms very

stable complexes with most metals.

HIN.

O+ _H+®

Scheme 1.1 The enolisation and ionisation of pentane-2,4-dione (acetylacetone)

The complex trichloro(pentane-2,4-dionato)titanium(IV)

has been fully

characterised by X-ray crystallography.

29 In the solid state it is dimeric and is

prepared by the direct reaction of TiCl4 with acetylacetone in a 1: 1 molar ratio. The

disubstituted product [TiC12(acac)2] has also been prepared, and has been assigned as

having the cis-configuration of the TiC12 (terminal) group.

CI O

CIý CITiO

O CI cl

ý0

CI

Figure 1.9 _{The dimeric structure of [TiC13(acac)]}

The organometallic chemistry of the Group 4 metals is very extensive. Many

organometallic compounds have been isolated with most of them containing the

cyclopentadienyl (Cp, C5H5) ligand. This subject is of major importance owing to the

facility with which certain organo-Group(IV) compounds catalyse the polymerisation

of a-olefins using Ziegler-Natta catalysis.

3° This general topic will now be discussed

in more detail in view of its relevance to the work researched within this thesis.

Ziegler-Natty _Catalysis

Just over forty years ago Karl Ziegler (1955) noticed that during experiments

to synthesise long-chain aluminium alkyls by treating aluminium triethyl with ethene

under pressure (`Aufbau reaction') transition metal halides had a dramatic effect on

the course of the reaction. ' He discovered that nickel salts led to the dimerisation of

ethene to butene, and more importantly that TiCl4 catalysed the polymerisation of

ethene to give a relatively high melting linear polymer. Guillo Natta then applied this

catalytic system to propene and discovered that it promoted the stereoselective

polymerisation of propene. 32 This use of metal halides activated by aluminium alkyls

to polymerise alkenes (Ziegler-Natta catalysis) is now one of the most important

industrial processes.

Polyethylene, as produced by Ziegler-Natta catalysis, is made up of long

chains of CH2 units which contain very few of the branches typical of polyethylene

made using free radical catalysts. However, with polypropylene three structural types

are possible

Isotactic

Syndiotactic

Atactic

Figure 1.10 _{Isomers of polypropylene}

These different isomers of polypropylene polymer have different properties.

Industrially it is desirable to have a polymer with a stereoregular structure (i. e.

isotactic or syndiotactic), and Ziegler-Natta catalysts are specifically designed to

produce these specific stereoregular types.

The mechanism of Ziegler-Natta Catalysis.

Since the initial discovery of this important class of catalysts, the precise

reaction pathway of the process has still not been fully established. It is generally

believed that the polymerisation process involves the formation of a complex between

the alkene and the active site of the catalyst, followed by a propagation step where

the added alkene extends the polymer chain.

The main belief is that propagation occurs at a metal-alkyl bond which could

be the transition metal alkyl, the activator alkyl, or an alkyl group which is bridging

between these two components. The Cossee-Arlman mechanism33 is now commonly

accepted as the mechanism for alkene polymerisation; the active species is a metal

alkyl with a vacant coordination site in a cis-position to the alkyl ligand:

0

----CI /Ti CI

CI

CI

AIEt3 CI

---- CI Ti Et

CI

CI

CH2Et

cl CI

/ Ti

El

CI

CI

vacant site

CHZ CH2

CH2Et

CHZ CH2 cl

---- CI Ti

II

CI

CI

H2C H2

I,

CI

---- CI Ti Et

CI

CI

No etc.

Figure 1.11

_{The Cossee-Arlman mechanism}

The alkene monomer coordinates to the transition metal at a vacant site. The

7t-coordination of the alkene monomer to the transition metal is shown in Figure 1.12.

This bonding was first developed by Dewar, Chatt and Duncanson where the

perpendicular orientation of the alkene situates the filled 1t and empty n* orbitals

properly for overlap with metal orbitals.

HH

,

I&M

HH

HH 1I

C OI

M .C

HH

(a) L-Mß donation (b) M-Ln back-donation

Figure 1.12 _{The Chatt-Dewar-Duncanson picture of bonding in a metal-olefin}

complex (Arrows show the direction of electron flow)

The alkyl group is then transferred to the bound alkene, where upon the resulting

alkyl group forms a a-bond to the transition metal atom. The complex then returns to

the initial state by exchange of the polymer chain and the vacant site, allowing the

polymerisation process to be repeated. The method of alkyl migration/transfer has

been studied in detail in metallocene complexes and these results are discussed in

more detail in this section (p. 19).

Stereoregulation in Propene Polymerisation Catalysts

A successful catalyst in propylene polymerisation is dependent upon its ability

to control the stereochemistry of the growth step, so that a crystalline, isotactic

polymer can be produced. The history of development of these heterogeneous

stereospecific catalysts falls into two main periods. The first period encompasses the

TiC13-based catalysts originally developed by Natta and co-workers ('first

generation' catalysts), along with a large number of `second generation' catalysts

based on TiC13 and modified with organic ligands which behave as Lewis bases. The

second period produced the highly stereospecific and productive `third generation'

catalysts. These latter catalysts involve the use of an inert support (such as

magnesium chloride) which stems from the observations that the bulk of the titanium

sites within the TiCl3 lattice are inactive.

Supported ('Third Generation') Catalysts.

Only a small number of titanium atoms are ideally situated to behave as active

sites. Thus TiCl3 itself may be considered as a self supported catalyst in which the

majority of the titanium atoms are within the bulk of the lattice, and therefore

inactive. The bulk TiCI3 may be replaced by an inert support, primarily magnesium

chloride, with active titanium centres being supported on the exposed surfaces.

The choice of components of a successful supported catalyst for the

polymerisation of propene is limited, and in practice a titanium(IV) compound is used

instead of TiCI3. Typically, these types of catalyst comprise of magnesium chloride,

an aromatic ester (e. g. ethyl benzoate), and titanium tetrachloride which is used in

conjunction with a trialkylaluminium compound combined with another aromatic

ester (e. g. AlEt3 with ethyl anisate).

Milling of the magnesium chloride increases the surface area and creates

disorder and defects in the lattice. Titanium tetrachloride is then absorbed onto the

surface giving a monolayer of active titanium sites. No other support for TiC14

functions as well as MgC12 which is probably a consequence of the comparability of

the ionic radii of Ti4± and Mg2+ which are very similar at 0.68 and 0.65Ä respectively.

If a Lewis base is present when the MgCl2 is milled, the surfaces are rapidly

complexed, thus preventing the reagglomeration of the MgC12 particles and increasing

the activity of the system even further. Although this may be achieved with a variety

of Lewis bases, high stereo specificity is only achieved by aromatic esters. The most

successful results have been obtained with the use of methyl and ethyl esters of

benzoic, toluic or anisic acids. The reason for this remains obscure but may well be

associated with their ability to impose a favourable steric and electronic arrangement

at the titanium centre.

Group 4 Metallocene Complexes and Catalysis.

Since the discovery of the TiCl4/AIEt3 Ziegler-Natta catalysts, the

polymerisation of a-olefins has developed into a major industry. Industrial processes

often use heterogeneous (or supported) catalysts which have been specifically

designed to be highly selective and efficient. However, as the catalysis takes place on

the edges and at dislocations of the support system, coupled with the titanium centre,

the resulting polymer has a broad molecular weight distribution

. 3' The properties of

these catalysts and the coordination geometry of the reaction centres is uncertain due

to the non-uniformity of active sites, and the limited information available on their

structural detail. 34

As a result it was hoped that homogeneous organometallic catalysts capable

of stereoselective a-olefin polymerisation would allow direct observations on the

active site involved, and hence the mechanisms of polymer growth and its

stereochemical control to be both determined and controlled. 35

After the synthesis of the first Group 4 metallocenes by Wilkinson et al, 36 it

was reported that homogeneous reaction mixtures of dicyclopentadienyl titanium

dichloride [Cp2TiCI2] (Figure 1.1) and diethyl aluminium chloride could be used as

homogeneous Ziegler catalysts, and these catalysts do indeed possess moderate

activity. 37'38 However these homogeneous catalysts did not show any activity for

polymerisation with propene. Another problem which was found to exist with these

catalysts was that they were easily reduced to an inactive Ti(III) species and as a

result were unable to compete with the highly active heterogeneous catalysts.

A major advantage with these metallocene based catalysts was their solubility,

since this allowed better kinetic data to be obtained, leading to proposals of the

possible reaction pathway.

As explained previously such studies involving

heterogeneous catalysts are understandably less well defined.

Consequently

numerous studies were aimed at the identification of reaction intermediates and

reaction mechanisms of these homogeneous catalysts. Breslow and Newburg39

suggested that in their [Cp2TiCl2] / [Et2AICI] system alkylation of the transition metal

is first achieved, to form [Cp2TiRCl] by ligand exchange with the alkyl aluminium

cocatalyst. This then forms a halide-bridged binuclear complex that has the ability to

react with ethene.

Cp Ti/CI

AIR2C1

Cp

Ti/CI

AIR CI

`2

2 ýR 2 ýR

(ethene)

g+ýCIAIRCl R2

Cpl ÄI

ýYý

$+ cl 6-

Cp2Tiý 'AIR2C1

CH-CH

2 R

Figure 1.13

_{The pathway of polymerisation suggested by Breslow and Newburg}

It was envisaged that there was polarisation of the molecule with a partial positive

charge on the titanium and negative charge on the aluminium. This early metallocene

work by Breslow, and subsequent studies by Chien,

40

contributed to the verification

of the ideas put forward in the Cossee-Arlman mechanism for heterogeneous

Ziegler-Natta catalysis. " These studies did not however resolve the nature of the

active species. One possibility is that alkene insertion occurs into a bimetallic species,

in which an alkyl group or halogen bridges the titanium and aluminium centres. 4144

Another suggestion requires the formation of a truly ionic species [Cp2TiR]+ by

abstraction of a halide anion and its incorporation into an anion [R,, Cl4_,,

All-. 45 There

was no direst evidence for the existence of such a species until Eisch et a1'6

discovered that PhC = C-SiMe3 reacted with [TiC12Cp2] in the presence of A1C12Me

to give the cationic titanium vinyl complex (Figure 1.14).

1+ _AICI4 Ph

Me

CP2Ti

SiMe3

Figure 1.14 _{The cationic titanium vinyl complex.}

Subsequent studies by Jordan et al4' isolated the tetraphenyl borate salts of cations

such as [Cp2ZrCH3. THF]+ and [Cp2ZrCH2Ph.

THF]+ which polymerised ethylene

without the addition of any activator. These and related findings in the groups of

Bochmann, 48 Teuben, 4' and Taube5° established that (alkyl)metallocene cations are

crucial intermediates in homogeneous polymerisation catalysis. Such a complex has

two low lying unoccupied orbitals d, t and dß (Figure 1.15)51 where the d6 orbital acts

as the acceptor for the incoming alkene molecule.

d71

111k

Figure 1.15

Showing the two low lying unoccupied orbitals d7t and da

The chain growth mechanism as well as the structure of the transition state in

the polymerisation reaction have attracted considerable attention. Brookhart et a152

suggested that the insertion of an olefin into the metal-alkyl bond is facilitated by an

agostic interaction of one of the a-H atoms of the metal-bound alkyl chain with the

metal centre of the Ziegler-Natta catalyst (Figure 1.16).